1. Field of the Invention
The present invention relates to an image forming apparatus including a light scanning device.
2. Description of the Related Art
Hitherto, there has been known an electrophotographic image forming apparatus in which a laser light (hereinafter referred to as a light beam) emitted from a light source is deflected by a rotary polygon mirror, and a surface of a photosensitive member is scanned by the deflected light beam to form an electrostatic latent image on the surface of the photosensitive member. The electrophotographic image forming apparatus is configured to form an image on a recording medium by use of an electrophotographic image forming process. Examples of the electrophotographic image forming apparatus include an electrophotographic copying machine (for example, a digital copying machine), an electrophotographic printer (for example, a color laser beam printer, a color LED printer, or the like), a multifunctional peripheral (MFP), a facsimile machine, and a word processor. The electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) is not limited to an image forming apparatus configured to form a monochrome image, and but may include a color image forming apparatus.
The image forming apparatus is configured to execute automatic power control (hereinafter abbreviated to as APC) in order to control the light intensity of the light beam emitted from a light scanning device in order that the light intensity matches a target light intensity. With the APC, the light beam emitted from the light source enters an optical sensor from which an output is fed back to control a drive current to be supplied to the light source based on the output from the optical sensor. By executing the APC, the light intensity of the light beam is controlled to match the target light intensity.
FIG. 7 is a schematic view of a package 510 of an edge emitting laser (EEL) 500 serving as the light source. The edge emitting laser 500 emits respective light beams 503 and 504 in two directions indicated by the arrows A and B from half mirrors 502 formed on both end surfaces of a semiconductor laser chip 501. The light beam 503 emitted in the direction indicated by the arrow A is referred to as a front light, and the light beam 504 emitted in the direction indicated by the arrow B is referred to as a rear light. The front light 503 is guided to the surface of the photosensitive member to form an electrostatic latent image on the surface of the photosensitive member.
FIG. 8 is a graph showing a relationship between the light intensity of the front light 503 and the light intensity of the rear light 504 of the edge emitting laser 500. As shown in FIG. 8, there is a correlation between the light intensity of the front light 503 and the light intensity of the rear light 504. The light intensity of the rear light 504 linearly changes in accordance with the light intensity of the front light 503.
With use of this characteristic, for the edge emitting laser 500, an optical sensor 505 (photoelectric conversion element) is provided in the package 510 as illustrated in FIG. 7 so that the rear light 504 enters the optical sensor 505 for execution of the APC.
By the way, in a case of a surface emitting laser (vertical cavity surface emitting laser: VCSEL), as compared to the edge emitting laser 500, a plurality of light emitting elements may be easily arrayed on one surface of a single semiconductor laser chip. The surface emitting laser emits a light beam in one direction. Therefore, an image forming apparatus comprising the surface emitting laser is provided with, as configurations for executing the APC, a beam splitter configured to split a light beam emitted from the surface emitting laser into two light beams, and an optical sensor arranged separately from the semiconductor laser so that one of the split light beams enters the optical sensor. The image forming apparatus executes the APC based on the light receiving result of the light beam entering the optical sensor (Japanese Patent Application Laid-Open No. 2004-153148).
In the image forming apparatus including the surface emitting laser, the light beam split by the beam splitter enters the optical sensor, and hence, as compared to the image forming apparatus including the edge emitting laser, the light intensity of the light beam that can enter the optical sensor is lower.
In general, a photodiode is used as the optical sensor. The photodiode outputs a current corresponding to the intensity of the incident light. However, the photodiode also has a characteristic of outputting a dark current when the photodiode is not irradiated by light. The amount of the dark current to be generated by the photodiode changes depending on temperature.
In general, the dark current has an extremely small current value such as several nanoamperes to several hundred nanoamperes. Therefore, when a light intensity of the light beam entering the photodiode is large to a certain extent, the dark current is negligible. However, when the APC is executed for the surface emitting laser, the light intensity of the light beam entering the photodiode is very small as described above, and hence the influence of the dark current cannot be neglected.
Further, in the image forming apparatus, in order to improve the output responsiveness of the light beam, in general, a predetermined bias current is supplied in advance to the semiconductor laser serving as the light source.
FIG. 9 is a graph showing a relationship between a current supplied to a semiconductor laser and an output light intensity. In order to improve the output responsiveness of the light beam, a bias current Ib is supplied to the semiconductor laser. The bias current Ib is smaller than a current value (threshold current) Ith at which the semiconductor laser starts laser oscillation. When the bias current Ib is supplied to the semiconductor laser, the semiconductor laser enters a spontaneous emission state. In the spontaneous emission state, the semiconductor laser emits low light (hereinafter referred to as “LED light”) having a wide wavelength range similarly to the LED. When the number of the light emitting elements is small, the light intensity of the LED light does not reach a level to change the potential of the photosensitive member. Therefore, during image formation, the bias current Ib is supplied to each of the light emitting elements of the semiconductor laser, and the semiconductor laser is driven while superimposing a current onto the bias current Ib to excite the laser oscillation.
However, the recent image forming apparatus tends to have a semiconductor laser including a plurality of light emitting elements in order to increase speed and image resolution. When the number of the light emitting elements increases, the influence of the light intensity of the LED light of the semiconductor laser cannot be neglected.
As a general technology of removing an offset component (DC component) such as the dark current and the LED light, there is known a system of extracting only an AC component by connecting a capacitor in series to an output terminal of the optical sensor (light receiving element). According to this technology, if the relationship between a time period in which light enters the optical sensor and a time period in which light does not enter the optical sensor is always constant, only the AC component can be extracted with high accuracy by adjusting the capacitance of the capacitor.
However, in the image forming apparatus including the plurality of light emitting elements, the APC is executed in a time sharing manner for each light beam during a period (non-image forming period) in which the light beam is emitted to an area (non-image area) outside an image forming area on the photosensitive member. Therefore, the time period for charging or discharging the capacitor is not always constant, and thus it is difficult to remove the offset component with high accuracy in the conventional art.